1. Field of the Invention
The present invention relates to a semiconductor optical device in which an EA modulator and a DFB laser used as a light source of the EA modulator are monolithically integrated and a method of fabricating the same, and, more particularly, relates to the semiconductor optical device that excels in modulation characteristics and includes the EA modulator most suitable for optical communications.
2. Description of the Related Art
A semiconductor optical device is developed and is put to practical use including an integrated optical modulator and semiconductor laser device in which an optical modulator and a single longitudinal mode semiconductor laser device used as a light source of the optical modulator are monolithically integrated.
As one of such integrated semiconductor optical devices, a semiconductor optical device is attracting attention that includes an electroabsorption type optical modulator (hereinafter referred to as an xe2x80x9cEA modulatorxe2x80x9d) that employs a change in the absorption coefficient by an electric field to be used as an optical modulator and a distributed feedback semiconductor laser device (hereinafter referred to as a xe2x80x9cDFB laserxe2x80x9d) to be used as a light source of the EA modulator.
The structure of a conventional EA modulator-DFB-laser-integrated semiconductor optical device (hereinafter referred to as an xe2x80x9cEA-DFBxe2x80x9d) will be described.
The conventional EA-DFB is a semiconductor optical device that includes a DFB laser and an EA modulator both of which are GaInAsP-based SI-BH (semi-insulated buried-hetero) types and in which a hetero-junction structure including a multi-quantum well structure in a semi-insulating layer is buried, and the DFB laser and the EA modulator are monolithically integrated on an n-InP substrate coaxially in a waveguide direction.
In a DFB laser area of the n-InP substrate common to the EA modulator, the DFB laser has a stacked structure including an n-InP lower cladding layer having a thickness of 100 nm, an SCH (Separate Confinement Heterostructure)-MQW (multi-quantum well) made of GaInAsP of which a band-gap wavelength xcexg is 1.55 xcexcm, a p-InP upper cladding layer having a thickness of 100 nm, a diffraction grating formed in a diffraction grating layer having a thickness of 10 nm and made of GaInAsP of which a band-gap wavelength xcexg is 1200 nm, a p-InP upper cladding layer having a thickness of 250 nm that includes a p-InP capping layer having a thickness of 10 nm, and a p-InP upper cladding layer having a thickness of 2000 nm, a p-GaInAs contact layer having a thickness of 300 nm that are common to the EA modulator.
The top surface of the lower cladding layer, the SCH-MQW, the upper cladding layer, the diffraction grating, the upper cladding layer including the p-InP capping layer, the upper cladding layer, and the contact layer of the stacked structure form a mesa structure.
The mesa structure is sandwiched by Fe-doped semi-insulating InP layers (hereinafter referred to as xe2x80x9cFe-InP layerxe2x80x9d) common to the EA modulator.
A common passivation film made of a SiN film is formed on the Fe-InP layers on both sides of the mesa structure excluding a window on the contact layer. A p-electrode is formed on the contact layer disposing the window therebetween, and a common n-electrode is formed on the bottom surface of the common n-InP substrate.
On an EA modulator area on the n-InP substrate common to the DFB laser, the EA modulator has a stacked structure including an n-InP buffer layer having a thickness of 50 nm, an SCH-MQW made of GaInAsP of which a band-gap wavelength xcexg is 1.52 xcexcm, a p-InP upper cladding layer having a thickness of 200 nm, and a p-InP upper cladding layer having a thickness of 2000 nm and a p-GaInAs contact layer having a thickness of 300 nm that are common to the DFB laser.
The top surface of the n-InP buffer layer, the SCH-MQW, the upper cladding layer, the upper cladding layer, and the contact layer 42 of the stacked structure is formed as a mesa structure. The semi-insulating Fe-InP layer common to the DFB laser is buried in both sides of the mesa structure.
The common passivation film made of a SiN film is formed on the Fe-InP layer on both sides of the mesa structure excluding a window on the contact layer.
A p-electrode is formed inside of the window on the contact layer, and a common n-electrode is formed on the bottom surface of the common n-InP substrate.
Then, a method for fabricating the above conventional EA-DFB will be described.
First, the GaInAsP-based DFB-LD structure is formed on the whole area of the n-InP substrate including the DFB laser area and the EA modulator area.
In other words, on the whole area of the n-InP substrate, the n-InP lower cladding layer, the SCH-MQW, the p-InP upper cladding layer, the diffraction grating layer, and the p-InP cap layer are epitaxially grown according to, for example, the MOCVD method.
Thereafter, the cap layer and the diffraction grating layer are etched to form the diffraction grating, and then the p-InP upper cladding layer is epitaxially grown to bury the diffraction grating, and the stacked structure including the cladding layer is formed on the diffraction grating.
Then, an SiN mask for covering the stacked structure of the DFB laser area is formed, and the stacked structure formed in the EA modulator area that is not covered with the mask is etched to expose the n-InP substrate.
Thereafter, the GaInAsP-based EA modulator structure is selectively grown on the n-InP substrate in the exposed area of the optical modulator area. In other words, the n-InP buffer layer, the SCH-MQW, and the p-InP upper cladding layer are epitaxially grown on the n-InP substrate according to, for example, the MOCVD method, thereby forming the stacked structure.
After the SiN mask in the DFB laser area is removed, the p-InP upper cladding layer and the pGaInAs contact layer are epitaxially grown on the whole surface of the substrate.
Thereafter, striped SiN masks having a width of 2 xcexcm are consecutively formed on the stacked structure of the DFB laser area and on the stacked structure of the EA modulator area, and then dry etching is performed by using the masks.
Thereby, the mesa structure including the top section of the lower cladding layer, the SCH-MQW, the upper cladding layer, the diffraction grating, the upper cladding layer having the p-InP cap layer, the upper cladding layer, and the contact layer is formed in the DFB laser area.
On the other hand, the mesa structure including the top section of the n-InP buffer layer, the SCH-MQW, the p-InP upper cladding layer, the upper cladding layer, and the contact layer is formed in the EA modulator area.
Thereafter, using the SiN masks as selective-growth masks, a semi-insulating Fe-InP current blocking layer is subjected to buried growth to fill the spaces in both sides of the mesa structure therewith.
The EA-DFB can be fabricated by forming the passivation film, the p-electrodes and the n-electrode.
The above conventional EA-DFB exhibits excellent modulation characteristics. However, in order to satisfy the demand of high-capacity/high-speed communications in the optical communication field, the EA-DFB has been demanded that excels in temperature characteristics and shows excellent high-speed modulation characteristics.
In one aspect of the present invention (hereinafter referred to as xe2x80x9cfirst inventionxe2x80x9d), a monolithic semiconductor optical device is provided including: a semiconductor substrate and an electric absorption modulator (EA modulator) including a quantum well structure having AlGaInAs-based material and a width larger than a width of an optical mode field as viewed perpendicular to an optical axis of said EA modulator, and a distributed feedback laser device (DFB laser) including a quantum well structure formed in an EA modulator formation area and in a DFB laser formation area, respectively, on the semiconductor substrate; wherein either of the EA modulator and the DFB laser is firstly formed in the EA modulator formation area or in the DFB laser formation area by etching the other of the EA modulator and the DFB laser to expose the semiconductor substrate in the DFB laser formation area or in the EA modulator formation area, and then the other of the EA modulator and the DFB laser is formed in the EA modulator formation area or in the DFB laser formation area.
In accordance with the first invention, the semiconductor optical device including the EA modulator excellent in the temperature characteristic and the modulation characteristic can be realized by forming the configuration including the EA modulator having the quantum-well-structure active layer made of the AlGaInAs-based material and the specific compound semiconductor stacked structure on the active layer, and the DFB laser that includes the quantum-well-structure active layer made of the GaInAsP-based material that is formed as a BH structure.
In another aspect of the present invention (hereinafter referred as xe2x80x9csecond inventionxe2x80x9d), a method for fabricating a semiconductor optical device is provided including the steps of: forming a first stacked structure including either of a DFB laser having a quantum-well-structure active layer made of a GaInAsP-based material or EA modulator having a quantum-well-structure active layer made of an AlGaInAs-based material in a DFB laser formation area and in an EA modulator formation area, respectively, on a semiconductor substrate; forming a second stacked structure by etching the area corresponding to the other of the DFB laser or the EA modulator to expose the substrate in this area; forming a third stacked structure including the other of the DFB laser or the EA modulator on the exposed substrate; and forming a mesa stripe by simultaneously etching the second and third stacked structures.
In accordance with the second invention, the suitable method for fabricating the semiconductor optical device of the invention can be realized. In other words, by the dry etching for the mesa formation that uses etchant of methane mixed gas or bromine-containing etchant, the DFB-LD area made of a GaInAsP-based material can reach halfway of the substrate, however, only the upper side of the EA modulator area made of an AlGaInAs-based material is etched because the AlGaInAs is not etched. Therefore, the BH structure and the buried ridge structure can be easily integrated.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.